Continuous fiber reinforced composites are extensively used in both primary and secondary aircraft components for a variety of applications where light weight, higher strength and corrosion resistance are primary concerns. Composites are typically composed of fine carbon fibers that are oriented at certain directions and surrounded in a supportive polymer matrix. Since the plies of the composite material are arranged at a variety of angles, and depending upon the direction of major loading, the resultant structure is typically a stacked laminated structure, which is highly anisotropic and heterogeneous. A significant portion of the composite structure is fabricated as near net-shape, but is drilled in order to facilitate joining of components using mechanical fasteners. Drilling fastener holes in composite does not compare to the uniformity of aluminum or steel since individual carbon fibers fracture at irregular angles and form microscopic voids between the fastener and the hole. As the cutting tool wears down, there is an increase of surface chipping and an increase in the amount of uncut fibers or resin and delamination. The composite microstructure containing such defects is referred to as “machining-induced micro texture.”
In addition to their machining challenges, composite structures in aircrafts are more susceptible to lightning damage compared to metallic structures. Metallic materials, such as aluminum, are very conductive and are able to dissipate the high currents resulting from a lightning strike. Carbon fibers are 100 times more resistive than aluminum to the flow of current. Similarly epoxy, which is often used as a matrix in conjunction with carbon fibers, is 1 million times more resistive than aluminum. The composite structural sections of an aircraft often behave like anisotropic electrical conductors. Consequently, lightning protection of a composite structure is more complex, due to the intrinsic high resistance of carbon fibers and epoxy, the multi-layer construction, and the anisotropic nature of the structure. Some estimates indicate that, on average, each commercial aircraft in service is struck by lightning at least once per year. Aircraft flying in and around thunderstorms are often subjected to direct lightning strikes as well as to nearby lightning strikes, which may produce corona and streamer formations on the aircraft. In such cases, the lightning discharge typically originates at the aircraft and extends outward from the aircraft. While the discharge is occurring, the point of attachment moves from the nose of the aircraft and into the various panels that compromise the skin of the aircraft. The discharge usually leaves the aircraft structure through the empennage.
The protection of aircraft fuel systems against fuel vapor ignition due to lightning is even more critical. Since commercial aircraft contain relatively large amounts of fuel and also include very sensitive electronic equipment, they are required to comply with a specific set of requirements related to the lightning strike protection in order to be certified for operation. It is a well-known fact that fasteners are often the primary pathways for the conduction of the lightning currents from skin of the aircraft to supporting structures such as spars or ribs, and poor electrical contact between the fastener body and the parts of the structure can lead to detrimental fastener arcing or sparking.
To avoid the potential for ignition at the fastener/composite structure interface, some aircraft use fasteners which are in intimate contact with the fastener hole. Intimate contact between bare metallic fasteners and the hole in the composite structure has been known to be the best condition for electrical current dissipation. One approach to achieve fastener-to-composite hole intimacy is to use a sleeved fastener. This approach involves first inserting a close fitting sleeve in the hole. An interference-fit pin is then pulled into the sleeve. This expands the sleeve to bring it in contact with the wall of the hole in the composite structure. Although the sleeve substantially reduces the gap between the fastener and composite structure, it cannot eliminate the small gaps created due to the presence of drilling induced texture across the composite inner-hole surface. This machining induced texture also entraps excess sealant, an insulating material, inhibiting the intimate contact between the sleeve and the hole. This situation becomes even worse as the cutting tool wears, resulting in more and larger machining induced defects.
In order to avoid this condition, the current must dissipate through the carbon fibers perpendicular to the fastener hole. If the fastener is not in intimate contact with the inside of the hole, the instantaneous heat energy ionizes the air in the gap and creates arc plasma that blows out in the form of a spark. The intrinsic high conductivity of metallic fasteners and the large number of fasteners used in aircraft construction combine to create a condition of a high probability of lightning attachment to fasteners.